It is well known to provide a two-wire telephone line interface circuit in which the outputs of a line driver circuit are coupled to the tip and ring wires of a telephone line via a sensing circuit which is responsive to current on the wires. The sensing circuit enables the hook status of the subscriber's telephone to be determined by monitoring the loop or differential direct current on the wires, as well as providing a path for a.c. (e.g. voice) signals to the telephone central office (C.O.). In order to provide for ringing signals to be supplied to the line for activating a ringer of the subscriber's telephone, generally contacts of a ringing relay are provided on the output side of the line driver circuit, to connect the tip and ring wires to ground and to a ringing voltage generator respectively and to interrupt their connections to the line driver circuit outputs so that the line driver circuit is not exposed to the high ringing voltage.
In such an arrangement costs are reduced by sharing the ringing voltage generator among many line interface circuits, but as a consequence a ringing voltage generator may not always be immediately available for supplying a ringing signal to a line when desired, and the provision of different ringing signals (different frequencies or different ring signal sequences) is complicated. Similar arrangements may be provided for other forms of high voltage signalling, such as coin and message waiting signalling, with similar disadvantages.
In Bolus et al. U.S. Pat. No. 4,431,868 issued Feb. 14, 1984 and entitled "Solid State Telephone Line Interface Circuit With Ringing Capability" there is described a line interface circuit in which a low-level ringing reference signal can be supplied to the input side of the line driver circuit to be amplified thereby to produce the desired high voltage ringing signal at the outputs of the line driver circuit, and at the same time the supply voltage rails of the line driver circuit are controlled to follow (with an offset) the instantaneous values of the ringing signal waveform thereby to reduce power dissipation in the line driver circuit. In this arrangement a voltage generator for producing the line driver circuit supply voltages is controlled in dependence upon the voltages sensed at the outputs of the line driver circuit.
With evolution of line interface circuits, and especially with increasing numbers and types of service (e.g. ringing frequencies and sequences) being desired by and available to telephone subscribers, it is becoming increasingly necessary to provide equipment such as line interface circuits which can be used in a versatile manner to provide various features or services as these may be desired.
An object of this invention is to provide an improved line interface circuit.